Vole with an injury above its eye.
Found this nibbled snail shell in the middle of a water vole feeding platform. That's no guarantee a water vole left it there, but while I was watching, one came and picked up the shell briefly and dropped it. I've still never actually seen voles of any species eating meat but I've come across so many snail shells in water vole areas, I really think they do. Pregnant female water voles have been recorded as eating fish.